President Barack Obama tried to reassure Britons on Friday that the special relationship between Washington and London would withstand an impending and likely messy divorce from Europe, as his administration scrambled to assess the fallout from a decision that left people stunned on both sides of the Atlantic.

As stock markets tumbled the day after the unexpected vote, Obama offered words of calm and continuity, saying the the shocking vote would disrupt neither the "enduring" bond between the US and the UK nor American support for the European alliance.

In a blitz of phone calls, Obama and his senior advisers called British and European officials to express support. 

Obama spoke with Prime Minister David Cameron and German Chancellor Angela Merkel separately to begin discussions on how Britain will extricate itself from the 27-member union, a process expected to take up to 2 years.

Obama said he came away from his call with Cameron "confident" that Britain is "committed to an orderly transition."

"One thing that will not change is the special relationship," between the US and the UK, Obama said in remarks at Stanford University.

The reassurance could not paper over what was clearly a blow to the president, who in one night saw his calls for unity rejected and his legacy in Europe reshaped.

Obama had strongly urged that the UK remain in the EU, and the decision significantly undermined his efforts to counter the isolationist viewpoints taking hold in many parts of the world.

The vote also ensured that, despite his high-profile aim of strengthening international alliances, he will leave office with the European Union diminished and under threat of further unraveling.